Recently, glass optical elements are widely used as a lens for digital cameras, a pickup lens for DVD, a lens for portable telephone cameras and a coupling lens for optical communication. As such glass optical elements, molded glass articles formed by press-molding glass material in a metal mold are frequently used.
One of production methods for such molded glass articles is known, in which method a glass pre-form having a predetermined weight and shape is made and the glass pre-form is heated together with a metal mold to a temperature, at which the glass becomes deformable, and pressed to be shaped (hereinafter also referred to as re-heat press method).
Conventionally, the glass pre-form to be used for the re-heat press method has been produced often by a mechanical process such as grinding and polishing. However, the method using grinding and polishing causes a problem that the method needs considerable labor and time. Therefore, a method is investigated in which molten glass is dropped onto a lower mold and cooled and solidified on the lower mold to prepare a glass pre-form without any mechanical processing.
Besides, another method is proposed in which a molten glass droplet is dropped onto a lower mold heated at a predetermined temperature and the molten glass droplet is shaped by pressing using the lower mold and an upper mold facing the lower mold to obtain a molded glass article (hereinafter referred to as droplet shaping method). This method is noted because the molten glass can be directly shaped from the molten glass droplet without repetition of heating and cooling of the metal mold so that the time necessary for whole molding can be considerably shortened.
However, when a glass droplet is dropped on the lower mold, a fine dent is formed, by hitting with the lower mold, near the central portion of the surface (contacting with the lower mold) of the glass droplet. Air in the dent has no escape route and is confined until the glass is cooled and solidified so that there is caused a problem that a dent (air retention) remains on the bottom surface of the glass pre-form or the molded glass article.
As a countermeasure to such a problem, a method for preventing the remaining of the air retention is proposed, in which the surface of the lower mold is roughened (Rmax of from 0.05 μm to 0.2 μm) so save the escaping rout for the air enclosed in the dent (for example, Patent Document 1).
Moreover, a lower mold is proposed, in which a coating layer containing a meltable layer is provided on the roughened base (Ra of from 0.005 μm to 0.05 μm) to prevent air from being retained and to make it easily recycled (for example, Patent Document 2).
Patent Document 1: Japanese Laid-Open Patent Application Publication No. H03-137031
Patent Document 2: Japanese Patent Application Patent Application No. 2005-272187